Sounds to me as if its a PLL loop filter problem. Check those capacitors
and resistors between the phase detector, the op amp if any, and the
VCO. Check board grounding also, e.g. tightness of board mounting
screws. I'd go with tantalum of the same as the marked value of the
aluminum electrolytics around that part of the circuit. Miniature
aluminum electrolytics are often not very good when new and age rapidly.
Last year there was a large number that blew up the first time they had
power applied on a very large number of computer mother boards made in
China. As if the electrolytics hadn't been formed, but just assembled.
When choosing miniature electrolytics I look for those rated for low ESR
and high temperature operation and find them a little better quality
than the more ordinary cheapest variety when checking for power factor
and leakage.
73, Jerry, K0CQ
--
Entire content copyright Dr. Gerald N. Johnson, electrical engineer.
Reproduction by permission only.
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